Found it: 12031(e). Only applies if they have reasonable cause to believe that you have a firearm. Basically, they have to see a gun, or a case that obviously contains one.

(e) In order to determine whether or not a firearm is loaded for the purpose of enforcing this section, peace officers are authorized to examine any firearm carried by anyone on his or her person or in a vehicle while in any public place or on any public street in an incorporated city or prohibited area of an unincorporated territory. Refusal to allow a peace officer to inspect a firearm pursuant to this section constitutes probable cause for arrest for violation of this section.

That's not even exactly allowing a search in itself. But in any case, this does not allow search without PC.

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